PURPOSE: To enable an excellent coding sound even by a low clock by enhancing efficiency of pitch tracking in long-term prediction in a sound coding system of about 8 to 4kb/s.

CONSTITUTION: A short-term prediction code to show a frequency characteristic is determined with every specific section of a sound signal, and a residual signal of a synthetic filter determined by the short-term prediction code is calculated by a inverse filter part 370. After the residual signal of a previous sound section is accumulated in a residual code book 390, a delay code to show the pitch correlation of the sound signal is tried by a delay code trial circuit 300, and a delay difference vector corresponding to a delay code is generated by a delay difference vector generating part 310 by using the residual code book. A delay vector is synthetically filtered by a synthetic filtering part 380, and a distance between the sound signal and a synthetic signal is calculated by a calculating part 330-1. The delay code whose delay value smoothly changes is found by a pitch pass estimating part 340-1 by using this distance.